1. Field of Invention
The present invention relates generally to the field of lifting fluids from well bores with a conveyor comprising a single continuous line or string of moving rope. More particularly, the present invention relates to apparatus and methods for compensating for and overcoming the inherent tendency of such a single rope conveyor to stretch and twist during operation.
2. Background Art
Systems used to remove liquids from new and existing well bores have changed very little over the years. Many variations have been proposed or tried, but most have failed or had limited success or applications due to physical limitations and/or a prohibitive cost.
The electrical submersible pump system has worked well for large volume producing wells and for very shallow "backyard variety" water wells, but is not economically feasible for the production of medium depth wells and/or marginal production wells.
The fill the gap, the downhole piston pump has been used almost exclusively around the world to produce the medium depth, marginal production wells. These pumps are powered by long strings of steel rods pushed in and out of the well bore by means of a powered eccentric system. Normally called walking beam pumps, these pumps have performed acceptably for many years and are still the industry standard. But walking beam pumping systems are expensive to purchase initially, and have very high maintenance costs. Another drawback of these pumps, in addition to high start-up and operating costs, is that cavitation occurs down hole on a regular basis. Cavitation in a down hole pump creates a turbulent movement of liquids in the well bore which limits the life of the pump and could disturb the formation, thus limiting the production from the well. Moreover, in an oil well with paraffins present, the valves in a walking beam pump can become plugged by the paraffins and cause pump failure, thus further limiting the usefulness of the walking beam pump in the oil patch.
Other conveyor-type systems have been tried and have failed due to design errors and limited applications. One of the most recent attempts at using a conveyor-type system is the Crafton Lift System as shown, for example, in U.S. Pat. No. 5,048,670. In that device, a single continuous rope conveyor travels through a tubular annulus. The utility of that system is limited, however, due to the elasticity characteristics of the tube and rope used as the carriers for lifting the liquids. In the absence of sufficient continuous tensioning force on the rope to "set" it, excess vibration could occur, even to the point of permitting natural harmonic vibrations to occur in the rope. As a result of this vibration, the fluids are quite literally shaken from the conveyor rope. Thus, in order to assure smooth operation and minimize fluid loss, it would be advantageous to reduce or eliminate any slack in the conveyor rope. Moreover, placing the required tubes in deep wells can be a very time-consuming and expensive proposition, in terms of both material costs (the tubes themselves) and the costs for installation, removal, and storage of the tubes. Furthermore, in the event that a tube breaks or becomes stuck down hole, the operator could be faced with a difficult and expensive fishing operation.